1. Sexual Life Cycle Made up of meiosis and fertilization Diploid cells
Somatic cells of adults have 2 sets of chromosomes
Haploid cells
Gametes have only 1 set of chromosomes
Offspring inherit genetic material from 2 parents

3. Sexual Life Cycle Stages
Life cycles of sexually reproducing organisms involve the alternation of haploid and diploid stages
Some life cycles include longer diploid phases, some include longer haploid phases
In most animals, diploid state dominates
Zygote first undergoes mitosis to produce diploid cells
Later in the life cycle, some of these diploid cells undergo meiosis to produce haploid gametes

5. Features of Meiosis Meiosis includes two rounds of division
Meiosis I and meiosis II
Each has prophase, metaphase, anaphase, and telophase stages
Synapsis
During early prophase I
Homologous chromosomes become closely associated
Includes formation of synaptonemal complexes
Formation also called tetrad or bivalents

6. Meiosis I vs Meiosis II
First meiotic division is termed the “reduction division”
Results in daughter cells that contain one homologue from each chromosome pair
No DNA replication between meiotic divisions
Second meiotic division does not further reduce the number of chromosomes
Separates the sister chromatids for each homologue

8. Prophase I
Chromosomes coil tighter and become visible, nuclear envelope disappears, spindle forms
Each chromosome composed of 2 sister chromatids
Synapsis
Homologues become closely associated
Crossing over occurs between nonsister chromatids

9. Crossing Over Genetic recombination between nonsister chromatids
Allows the homologues to exchange chromosomal material
Alleles of genes that were formerly on separate homologues can now be found on the same homologue
Chiasmata – site of crossing over
Contact maintained until anaphase I

11. Metaphase I
Terminal chiasmata hold homologues together following crossing over
Microtubules from opposite poles attach to each homologue
Not each sister chromatid as in mitosis
Homologues are aligned at the metaphase plate side-by-side
Orientation of each pair of homologues on the spindle is random

12. Random Orientation of Chromosomes During Metaphase I

13. Anaphase I Microtubules of the spindle shorten Chiasmata break
Homologues are separated from each other and move to opposite poles
Sister chromatids remain attached to each other at their centromeres
Each pole has a complete haploid set of chromosomes consisting of one member of each homologous pair
Independent assortment of maternal and paternal chromosomes

14. Telophase I Nuclear envelope re-forms around each daughter nucleus
Sister chromatids are no longer identical because of crossing over (prophase I)
Cytokinesis may or may not occur after telophase I
Meiosis II occurs after an interval of variable length

15. Meiosis II Resembles a mitotic division
Prophase II: nuclear envelopes dissolve and new spindle apparatus forms
Metaphase II: chromosomes align on metaphase plate
Anaphase II: sister chromatids are separated from each other
Telophase II: nuclear envelope re-forms around 4 sets of daughter chromosomes; cytokinesis follows

17. Final Result Four cells containing haploid sets of chromosomes
In animals, develop directly into gametes
sperm and eggs
In plants, fungi, and many protists, divide mitotically to produce greater number of gametes

18. Errors in Meiosis
Nondisjunction – failure of chromosomes to separate from one another as they move to opposite poles during either meiotic division
Aneuploid gametes – gametes with missing or extra chromosomes
Most common cause of spontaneous abortion in humans

19. Meiosis vs. Mitosis Meiosis is characterized by 4 features:
Synapsis and crossing over
Sister chromatids remain joined at their centromeres throughout meiosis I
Kinetochores of sister chromatids attach to the same pole in meiosis I
DNA replication is suppressed between meiosis I and meiosis II